Vakkiz the Windrager is one of four descendants of Terokk's ancient enemies who are being held by the Skettis talonpriests. You learn of thier existence from a tome named Adversarial Bloodlines. Originally, you were asked to retrieve Vakkiz's Scale if you intended to summon and defeat Terokk.

Open a skull pile at one of the red summoning portals in Skettis. If you have 10 Time-Lost Scrolls, you will be presented with a list of names. Select "Vakkiz the Windrager". The scrolls will be expended and Vakkiz will appear in a flash of lightning.

Bone Spray: At 40% HP, Vakkiz morphs into a bone dragon, similar in appearance to the Avatar of Hakkar. He then fires volleys that hit the entire party for ~500 damage, at the same rate the Lightning Breath occured. Every hit of Bone Spray applies a non-dispellable +5% damage debuff. Stacks to 20.

The primary concern in this fight is Bone Spray. It has practically limitless range and will quickly overwhelm your healer's mana, so do not permit your DPS to waste time. Throw everything you have at Vakkiz the moment he changes form.